The present invention relates to an apparatus useful for measuring the bubble point pressures of fluids and a method of using the same and, more particularly, to a multi-layered coaxial piezo-electric resonator for bubble point pressure measurement that is easily integrated with typical borehole fluid sampling tools.
A primary objective of borehole formation fluid sampling is to provide accurate determination of the physical and chemical properties of formation fluids. These properties can be strongly affected by temperature and pressure. Therefore, the in-situ temperature and pressure of fluid samples should be maintained throughout the extraction, conveyance and analysis processes. Ideally, these conditions should be as close to the borehole in-situ environment as possible, or at least be kept within a sufficiently safe margin from certain thermodynamically critical values beyond which the formation fluid sample might be irreversibly altered.
One such critical formation fluid property is the bubble point pressure of crude oil (also referred to herein as the xe2x80x9cbubble pointxe2x80x9d). If the borehole pressure is allowed to drop below the bubble point during oil production, gas bubbles will form in the porous rock reservoir dramatically decreasing the oil phase relative permeability. Accordingly, a reliable determination of the bubble point pressure is vital for complete characterization of a reservoir zone and its subsequent commercial exploitation. Further, knowledge of the bubble point is also useful in determining the composition of the hydrocarbon mixture in the reservoir.
One traditional method (referred to as the P-V technique) of measuring the bubble point is to bring a sample to the surface to be sent to a laboratory (called a PVT Laboratory). There, the sample is placed in a cylinder. The volume of the cylinder is then increased using a piston and the pressure is monitored. Using this method, the bubble point is the pressure at which a break (knee) appears in the pressure versus volume (P-V) curve.
However, the P-V technique has several disadvantages. One major drawback is that it is time consuming to bring a fluid sample to the surface, transfer it to the (possibly distant) laboratory, and await the results. Typically, a delay of several weeks occurs between the time of fluid sampling and the receipt of the laboratory report log. By the time the laboratory report is received, it may be too late to obtain additional samples. Further, because samples may be altered by pressure and/or temperature changes when they are brought to the surface and sample composition can change as a result of imperfect transfer from sampling bottle to transportation bottle and then to laboratory apparatus, the P-V technique may be unreliable. Further limitations of this technique are: (1) only a few samples (typically six or fewer) can be transported to the surface on each tool run; and (2) high pressure, toxic, and potentially explosive samples must be transported, handled, and disposed of, creating numerous potential health, safety and environmental problems.
Due to these problems, improved bubble point measurement techniques have been developed. In one such technique, bubble formation is observed in a cylinder by use of a sight glass. In this manner, bubbles are detected visually. Because the bubble point is associated with the attenuation of a light beam, more sophisticated methods measure the transmission of near infrared light.
Several additional methods have been proposed in which the bubble point pressure measurement can be made in-situ using a borehole tool. Existing formation sampling tools, such as the MDT by Schlumberger, employ a mechanically actuated expanding volume to control the draw-down pressure during fluid sampling and an optical analyzer to detect the first occurrence of gas bubbles. However, this measurement process can be time-consuming and inconsistent and has the attendant risk of losing the tool in the borehole.
More recently, cavitation has been identified as an ideal method for determining bubble point pressure in-situ using a borehole tool. As discussed in commonly owned U.S. Pat. No. 6,128,949 to Kleinberg (the ""949 Patent), acoustic cavitation refers to the generation of low-pressure regions in a liquid, which induces the evolution of gas bubbles. It has been established that for a liquid near the bubble point (i.e., the point at which bubbles are thermodynamically stable but form slowly), modest localized pressure reductions, such as are induced by acoustic waves, can lead to efficient evolution of bubbles. The ""949 Patent is incorporated herein by reference in its entirety.
Conventional ultrasonic transducers do not operate well in the 175xc2x0 C. and 20,000 psi borehole environment. Typical commercially available high-power ultrasonic transducers, due to less robust design, fabrication and construction criteria, are intended for use at surface pressures and temperatures and have not been adapted for use to nucleate bubbles in borehole-like conditions. Accordingly, there exists a need for an ultrasonic transducer suitable for use in a borehole tool that is capable of withstanding the harsh borehole environment and that is capable of nucleating bubbles in static pressures above the bubble point. While some drilling environments may exceed these operating conditions, these parameters generally are considered a baseline for the design of an ideal device.
Further, it is desirable to use a tool that does not impact, or only minimally impacts, the existing flow operation of the fluid sampling mechanism. For example, it is preferred to use a design that does not require an enlargement of the flow cross-section in mid-stream. Furthermore, the ideal design should not require a large sampling volume that is filled and evacuated separately and repeatedly.
Accordingly, one object of the present invention is to provide a device that can efficiently and advantageously induce and detect bubble formation at a significant static pressure margin above the bubble point pressure.
One further object of the present invention is to provide a device that is operable at temperatures and pressures up to 175xc2x0 C. and 20,000 psi.
A further object of the present invention is to provide a device whose design will integrate seamlessly into the flow-line structure of a host tool and is power-efficient.
Yet another object of the present invention is to provide an ultrasonic transducer/resonator suitable for use in the method of the ""949 Patent.
And, yet another object of the present invention is to provide a borehole bubble point measuring apparatus that has no moving parts and, therefore, is not prone to failure in a xe2x80x9cdirtyxe2x80x9d borehole environment.
The present invention discloses an apparatus useful for nucleating bubbles and allowing the determination of the bubble point pressure in a borehole environment. In one embodiment, an acoustic resonator device is disclosed having one or more coaxial layers forming a central conduit in fluid communication with a borehole formation fluid sampling means, wherein at least one of the coaxial layers is comprised of an electro-acoustic transducer material. While this device is preferably designed to be in fluid communication with a host tool to allow in-situ sampling and bubble point determination, it may be used in surface systems on samples under borehole like conditions. The device may be used in a tool using a captured volume sampling technique or a flow-line sampling technique (e.g. a portion of the fluid moving within a flow-line).
Also disclosed is an in-situ method of fluid analysis in the borehole of a well for determining phase characteristics of a formation fluid comprising the steps of: (a) withdrawing a formation fluid sample using a formation sampling tool equipped with an acoustic cavitation device having one or more coaxial layers forming a central conduit, wherein at least one of the one or more coaxial layers is comprised of an electro-acoustic transducer material; (b) activating the electro-acoustic transducer material thereby nucleating bubble formation in the sample; (c) detecting an onset of bubble formation within the central conduit; and (d) measuring pressure of fluid at the onset of bubble formation in accordance with step (c).
The present invention may be used to induce bubble nucleation in nearly any sample wherein the anticipated bubble point pressure is lower than the static sample pressure (for example tens to hundreds, and possibly even thousands, of psi difference). Most preferably, this device is suited to induce bubble nucleation under borehole-like conditions. In this embodiment of the present invention, a fluid sample is obtained, wherein the fluid sample has a static pressure higher than an expected bubble point pressure (in some cases possibly thousands of psi higher), using a sampling tool equipped with an acoustic cavitation device having one or more coaxial layers forming a central conduit, wherein at least one of the coaxial layers is comprised of an electro-acoustic transducer material. The electro-acoustic transducer material is activated to nucleate bubbles in the sample. Bubble formation may then be detected within the central conduit and the pressure of fluid measured at the onset of bubble formation.
Further features and applications of the present invention will become more readily apparent from the figures and detailed description that follows.